Mechanical and Physical Properties of Microcellular Starch-Based

Chapter 2 Mechanical and Physical

Properties

of Microcellular Starch-Based Foams Downloaded via TUFTS UNIV on July 14, 2018 at 13:28:57 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Formed from Gels G . M . Glenn, W. J . Orts, R. Buttery, and D. Stern Western Regional Research Center, Agricultural Research Service, United States Department of Agriculture, 800 Buchanan Street, Albany, C A 94710

Microcellular foams can be formed from rigid aqueous gels of starch. Foams were made from starch gels by exchanging the aqueous phase with ethanol before air-drying. The foam product had an extremelyfinemicrostructure comprised of starch granule remnants embedded in a fibrous network. The starch foams had a moderate density (0.14-0.37 g/cm ), low thermal conductivity (0.024-0.040 W/mK) and high compressive strength. The foams exhibited the capacity to adsorb polar compounds and alkylpyrazines. When the foams were compression molded, a starch plastic was formed with a tensile strength of more than 12 MPa. Tensile strength and the elongation to break more than doubled in the starch plastics when 33% cellulose fiber was incorporated in the formulation. X-ray diffractograms revealed only small amounts of recrystallization in starch foams and plastics after three years of aging. 3

Introduction Starch is the least expensive and most abundant worldwide food commodity (7). Its low cost and wide availability from year to year have made starch attractive as an industrial raw material. More than 4.5 billion pounds of starch are used in the United States for industrial applications (2). Industrial products made from starch include chemicals derivedfromfermentation processes, adhesives, sizing products, and soil conditioners (7-7). Starch is used on a limited scale for single-use disposable 42

U.S. government work. Published 2001 American Chemical Society Gross and Scholz; Biopolymers from Polysaccharides and Agroproteins ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

43 products that replace some petroleum-based products. For instance, various starchbased resins have been developed for making films or injection molded articles (8). Starch has also been used i n making polymeric foams (9-11). The best example is extruded starch-based foams that have gained a considerable portion of the loose fill packaging market. Polymeric foams consist of a solid polymer matrix formed into discrete elements or empty cells and a gas phase that fills the void space (12-13). Polymeric foams may be categorized based on cell size as macrocellular or microcellular. L o w density, macrocellular starch-based foams, such as those used for loose-fill packaging, are made by extrusion using water/steam as a blowing agent (9-11). These foams typically have cell diameters ranging from 100 to 1000 μιη (9-10). Microcellular foams have cell diameters under ΙΟμιη and have unique properties that are of commercial interest (14-17). For instance, they may provide remarkable sound and thermal insulation and can reduce material density without compromising strength. Microcellular foams have been successfully made from a wide array of polymers using replication of removable pore formers, polymerization of inverse monomer emulsions, blowing/nucleation methods, and phase separation of polymer solutions (16,18). A blowing/nucleation method was reported for making microcellular plastic foams. The technique used C 0 as a blowing agent i n an extrusion process where the C 0 was injected into the extruder barrel (15,17,19-20). A similar process was reported for making C 0 expanded starch foams (21). A n aqueous starch slurry was processed just below 100 °C i n an extruder to prevent steam from forming and functioning as a blowing agent as the melt exited the die. Carbon dioxide was injected into the starch melt within the extruder barrel. The starch melt expanded and formed a solid foam as it exited the extruder die. The cells of the C 0 blown starch foam were quite uniform but were much larger than 10 μιη i n diameter and had thick cell walls (21). A second method of particular interest for making microcellular plastic foams involves solubilizing a high molecular weight, semiciystalline polymer i n a solvent that is then cooled to induce phase separation and produce a gel (16). A foam is formed i f the gel is rigid enough to withstand the compressive forces created by surface tension as the solvent evaporates. Starch, a high molecular weight, semiciystalline polymer, swells and hydrates i n excess water at temperatures above the gelatinization temperature. A s a gelatinized starch solution is cooled, a gelation process occurs i n which the starch molecules reassociate. The starch gel is not rigid enough to withstand the surface tension created by water evaporation i n the pores of the starch gel matrix and simply collapses into a film i f air-dried. However, Glenn and Irving (22) demonstrated that the gel structure of wheat and corn starches could be at least partially preserved by exchanging the water within the gel matrix with a solvent having a lower surface tension before proceeding with solvent evaporation. This chapter describes methods of making starch-based microcellular foams and describes their properties and potential applications. 2

2

2

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Gross and Scholz; Biopolymers from Polysaccharides and Agroproteins ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

44 Experimental

Foam Forming Process

Unmodified starches of wheat (Midsol 50, Midwest Grain Products, Atchison, K S ) and two from corn, a regular Dent corn starch (Melogel) and a high-amylose starch (Hylon Y D ) (National Starch and Chemical Company, Bridgewater, NJ) were used to prepare foams. The wheat and Dent corn starches were composed of ~28% amylose and 72% amylopectin. The high-amylose corn starch contained - 7 0 % amylose and 30% amylopectin. Starch Gelatinization. Aqueous suspensions (71) of wheat or Dent corn starches (8% w/w) were vigorously mixed while being heated i n a boiling water bath. Viscosity was monitored (Brookfield, Model R V T , Stoughton, M A ) to determine when peak viscosity was reached. High-amylose com starch had a much higher gelatinization temperature than wheat or Dent com starch and was prepared i n a pressure reactor (Paar Instrument Co., Moline, IL) equipped with a mixer and controller (model 4843). The aqueous suspension of high-amylose corn starch (8% w/w) was heated at 2 °C m i n up to 140 °C. The starch melt was then cooled to 90 °C using an internal cooling coil. 4

Slab Foams. The gelatinized starch melts were poured into slab molds and chilled (5 °C) overnight to promote gelation. The starch gels were removed from the molds and equilibrated 48 hr each i n successive baths of ethanol (40, 60, 70, 90% and three changes of 100% ethanol). During the ethanol equilibration process, the gels shrank and became very rigid. The changes i n the gel strength enabled them to withstand the compressive force created by surface tension at the air/solvent interface within the pores of the starch foam matrix. The slabs were dried i n air filtered through desiccant until the ethanol odor was no longer detected. A variation of the wheat starch foam was created by adding a bleached softwood pulp fiber (Leafwood, Georgia Pacific, 4% w/w) to the aqueous starch (8% w/w) suspension. The starch-fiber suspension was heated and processed identically to the gelatinized wheat starch melts containing no fiber. The dry weight concentration of fiber i n the foam was approximately 33%. A modified procedure was used to make foam from gels of high amylose corn starch. The gels were first equilibrated i n ethanol as previously described. However, only a small amount of shrinkage occurred i n the high-amylose starch gels and they did not gain sufficient compressive strength to withstand the compressive forces of surface tension created during air-drying. Consequently, the gels were


45 equilibrated i n a pressurized autoclave containing a second solvent (liquid C 0 ) of even lower surface tension. After several changes of liquid C 0 , the high amylose corn starch gels were air-dried by gradually evaporating the liquid C 0 and storing the foams i n a desiccator. 2

2

2

Foam Beads. A second processing technique was used to make beaded foam from wheat starch with a processing time much shorter than that of the foam slabs. A n aqueous suspension of wheat starch (8%, w/w) was gelatinized as previously described. The hot starch melt (90 °C) was injected under pressure (0.069 to 0.14 MPa) through nozzles with a 0.5 mm diameter into a stream of chilled (10 °C) vegetable cooking oil. The jet stream of starch melt quickly formed into small (0.25 m m to 1.0 mm) spherical beads at a distance of 6-8 m m from the nozzle. The beads were collected i n a chilled o i l stream and carried to a settling tank composed of two liquid phases approximately 15 cm each i n depth. The upper phase consisted of chilled vegetable oil and the lower phase consisted of a mixture of ethanol i n water (35% w/w). The beads were collected from the bottom of the settling tank and equilibrated 24 hrs each i n two changes of two volumes of 100% ethanol. The beads were dried i n air filtered through desiccant until the ethanol odor was no longer detected. The density, thermal conductivity and mechanical properties of the foams were determined as described earlier (22). A l l starch samples were equilibrated at 50% relative humidity. Starch Plastics. Starch foams and fiber/starch foams were prepared as previously described. Starch plastics and fiber/starch plastics were formed by pressing starch foams with approximately 69 M P a of force using an hydraulic press (23).

Characterization of Starch Products.

Physical and Mechanical Properties. Thermal conductivity was measured at a mean temperature of 22.7 °C according to standard methods ( A S T M C 177-85). Density was determined from measurements of sample weight and volume. The samples were tested i n compression and tension using a universal testing machine (model 4500, Instron Corp., Canton, M A ) . Two methods, porosimetry analysis using nitrogen gas and Horvat-Kawazoe plots using argon, were used to determine pore size distribution i n the submicron and Angstrom ranges, respectively. Pore size distribution measurements were performed by an independent laboratory (Porous Materials, Inc. Ithaca, N Y ) . The adsorbent properties of the foams were studied to determine their potential i n encapsulating chemicals. Experiments were performed i n which 0.5 g of starch foam beads were sealed i n a flask with a 5 μΐ sample of volatile compounds having different chemical properties. The adsorption properties of the starch foams


46 were compared to those of two commercial adsorbents; charcoal and Tenax. The amount of compound adsorbed was determined by the vapor pressure depression measured i n the headspace of the sealed flask using gas chromatography. X-ray Diffiactometry. X-ray diflractograms were obtained using a Phillips X'Pert M P D diflractometer employing C u Ka radiation. Diflractograms were obtained from raw wheat starch powder, aqueous gels (8%, w/w), gels that were oven dried (80 °C) and oven dried gels that had aged for four weeks at 50% relative humidity. Diflractograms were also obtained from starch foam slabs and starch plastics that were freshly made or aged for three years at room temperature and 50% relative humidity.

Results and Discussion The densities of the foams made of wheat, corn and high amylose corn starch gels ranged from approximately 0.14 g/cm to 0.37 g/cm (Figure 1). The foams made of high amylose corn starch had the lowest density (0.14- 0.20 g/cm ) while foams made of unmodified Dent corn starch had the highest range i n density (0.25-0.37 g/cm ). The range i n density (0.24-0.30 g/cm ) of wheat starch-based foams was intermediate. The density of wheat starch/fiber foams was nearly half the density of wheat starch foams containing no fiber (Table I). The lower density of the fiber/starch foam compared to the starch foam was attributable to the difference i n the amount of shrinkage during the ethanol equilibration process. The fiber/starch gels exhibited only a small degree of shrinkage compared to the starch foams. The thermal conductivity of the wheat starch and fiber/starch foams was similar to that of a commercial beaded polystyrene (PS) foam (Table I) even though the densities of the starch and fiber/starch foams were one order of magnitude higher than the density of the PS foam. The low thermal conductivity observed i n the starch foams i n spite of their relatively high density i n comparison to PS foam may be explained by their microstructure. The PS foam had a closed œ i l structure with cells approximately 100 μιη i n diameter. The matrix of the starch and fiber/starch foams is composed of remnants of starch granules and many pores that are smaller than 2 μιη (Figure 2). A small pore size i n the foam matrix effectively reduces the amount of heat transfer by convection and conduction compared to larger foam pore sizes (24). The data support that relative to the PS foam, the effect of higher densities of the starch and fiber/starch foams on thermal conductivity was offset by a smaller pore size. 3

3

3

3

3

The mechanical properties of the fiber/wheat starch foams were more similar to the properties of PS foams than the wheat starch foams (Figure 3). The compressive strength and compressive modulus for the fiber/starch and PS foams were both approximately one order of magnitude smaller than that of the wheat starch foam containing no fiber (Table I). The wheat starch foams typically had a yield point i n the range of 0.4 to 0.6 M P a . Beyond the yield point, the stress


47

SIVI

ω ω m oc κ(0 LU ce α. CO til οέ 0. S ο ϋ

1.6 1.4 1.2 1 0.8 0.6 0.4

-r

---

0.2 0 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

3

DENSITY (g/cm ) Figure 1. The compressive strength of starch foams at 10% deformation was related to foam density. Starch foams made of high amylose corn starch ( • ) , had the lowest relative densities. The range i n density of foams made of regular corn starch ( A ) was greater than the density range for the wheat starch foams (O).

Figure 2. Scanning electron micrographs of wheat starch foams (a, b) and fiber/starch foams (c, d). The wheat starch foam (a, b) contained numerous starch granule remnants interspersed i n a fibrous matrix. Most of the pore diameters were smaller than 2 μιη. The fiber/starch foam (c, d) had a lower density and a more porous matrix. Notice the cellulose fiber embedded i n the starch matrix (c. d). Magnification bars: a, c = 100 μιη; b, d = 5 μιη.

American Chemical Society Library 1155 16th St.,from H.W. Gross and Scholz; Biopolymers Polysaccharides and Agroproteins ACS Symposium Series; American DC Chemical Society: Washington, DC, 2001. Washington, 20036

48

1

Ο

2

4

6

8

10

Strain (%) Figure 3. Compressive stress/strain curves for wheat starch, fiber/starch and polystyrene (PS) foams. The foams were tested to 10% strain. Note that the starch foams had a much higher modulus and compressive strength than either of the other two foams.


49 increased linearly with strain i n the range tested. The stiffiiess of the starch foam was much higher than that of the fiber/starch and PS foams as indicated by the compressive moduli (Table I).

Table L Density, compressive strength and modulus at 1 0 % deformation, and thermal conductivity of wheat starch foams both with and without 3 3 % (dry weight) softwood fiber. Data for polystyrene (PS) foam was included for comparison. Starch Foams

Fiber/Starch Foams

PS Foam

Density (g/cm )

3

0.28a*

0.16b

0.019c

Compressive Strength (MPa)

0.64a

0.086b

0.098b

Compressive Modulus (MPa)

11.2a

1.3b

3.6b

Thermal Conductivity (W/mk)

0.04a

0.039a

0.036a

Sample

a

Values within rows followed by a different letter are significantly different at the 95% confidence level.

Tensile tests of the wheat starch, fiber/starch and PS foams revealed that the wheat starch foam was much stronger i n compression than tension (Compare Tables I & Π). The tensile strength of the fiber/starch foams was similar i n both compression and tension. The tensile strength of the PS foam was more than one order of magnitude greater i n tension than compression and more than three times greater than that of the starch and fiber/starch foams (Table II). The comparatively high tensile strength of the PS foams is attributable to the structure of its matrix as well as the mechanical strength of PS polymer. Micrographs of the PS foams show a matrix of uniform cell size with thin sheet-like cell walls that form an interconnected matrix (22). In contrast, the matrix of the starch and fiber/starch foams was fibrous and less interconnectai (Figure 2). It was not surprising to find that the fiber component did not markedly increase the tensile strength of the fiber/starch foams compared to the starch foams because the fibers appeared loosely dispersed throughout the matrix and because the fiber/starch foams were less dense. The fiber did, however, increase the elongation to break and toughness of the fiber/starch compared to the wheat starch foams without fiber (Table II). The PS foams had a higher elongation to break than either the starch or fiber/starch foams but a lower tensile modulus than the starch foams (Table II).


50 Table I L Tensile strength, modulus, elongation and toughness of wheat starch, fiber/wheat starch and polystyrene (PS) foams. Sample

Starch Foams

Fiber/Starch Foams

Ts Foams

Tensile Strength (MPa)

0.094a

0.092a

0.32b

Tensile Modulus (MPa)

20.2a

6.73b

9.14b

Elongation (%)

0.77a

2.4b

4.8c

Toughness (J)

0.006a

0.028b

0.010a

a

a

Values within rows followed by a different letter are significantly different at the 95% confidence level.

One concern with pregelatinized starch products is that the mechanical properties may change with age (25). Dried films of gelatinized starch become hard and brittle over time (24). Shogren (26) reported that starch embrittlement over time was due to a combination of water loss and free volume relaxation. Starch may slowly recrystallize i f moisture levels are high enough to drop the glass transition temperature closer to the temperature of the starch sample (27-28). X-ray diflractometry has been used to help characterize the crystalline properties of starch powders (28). X-ray diflfractometry was used i n the present study, to determine the relative changes i n the ciystallinity of wheat starch over time. The X ray diffractogram for raw wheat starch (Figure 4) was typical for A-type starch (29). The diffractogram for aqueous starch gels consisted of a broad curve with no distinct peaks, indicative of a completely amorphous structure (Figure 4). The so-called "amorphous halo" or amorphous scattering occurred ~10-12 degrees higher compared to the curve for raw starch due to the high moisture content of the gel. The dried gel sample was nearly completely amorphous except for three small peaks (Figure 4). During a four-week period i n which the dried gel samples were stored at 50% relative humidity, the area of each of the three peaks increased (Figure 4). The data show that dried starch gels retrograded to some extent but the degree of crystallinity remained much lower than that of raw starch. In addition, the diffractogram of the dried gel was no longer an A-type pattern but was more similar to the B-type pattern found i n potato and high amylose corn starches (29-30). The B-type structure is


51

5

15

25

35

2 Thêta Figure 4. X-Ray diflractograms of raw wheat starch powder, an aqueous starch gel, dried gel, and a dried gel that had aged for 4 weeks at room temperature and 50% relative humidity. The dried gel and aged, dried gel had two peaks that occurred as similar angles (a=17.6, b=20.2) as two peaks found i n the starch powder. The curves were purposely offset to facilitate visual comparisons.


52 described as a more loosely packed assembly of helices with more inherent water than the A-type (31). The X-ray diflractograms of the starch foam slabs, which were prepared with ethanol, were compared to those of aged, dried gel that had not been exposed to ethanol (Figure 5). Although both the foams and the dried gel were made from starch gels, there was the possibility that the ethanol dehydration step used i n the process for making the foams altered the diffraction pattern. Starches generally contain 1% lipid or less and these lipids can form complexes with amylose with a unique X-ray diffraction pattern (32). However, the results of this study show that the X-ray diffraction pattern obtained from non-aged foams was similar to the X-ray diffraction pattern obtained from dried gels that were not exposed to ethanol (Figure 5). The peak heights were slightly greater i n the diffractogram of the starch foams that had been aged for three years (50% relative humidity, ~22 °C) but the change over time was only minor. There was no embrittlement or any other detectable change i n the mechanical properties of the foams as they aged (data not shown). However, it should be noted that an increase i n embrittlement would have been difficult to detect since the foams were brittle (low percent elongation to break) from the outset. The potential of the starch-based foams for commercial products based on their mechanical properties was considered. The low thermal conductivity and high compressive strength of the starch foams make them attractive for several commercial products. A vapor barrier could be applied for applications requiring moisture resistance. A second product of interest made from the starch foam or fiber/starch foam was a starch plastic (23). The tensile properties of the starch plastics were studied and compared to starch plastics that had been aged three years and a fiber/starch plastic (Table ΠΙ). The tensile strength of the starch plastic was approximately one half that of the fiber/starch plastic (Figure 6). The tensile strength of the starch plastics did not change significantly during a 3-year storage period. The tensile modulus and toughness were higher for the fiber/starch plastic than the starch plastic (Table HI). The tensile modulus of the starch plastic increased over time while toughness decreased. The elongation to break was nearly three times higher for the fiber/starch plastic compared to the starch plastic. The starch plastics embrittled slightly during storage as was evident by the lower elongation to break values (Table

m). A comparison of the mechanical properties of fiber/starch plastic and five commercial plastics revealed that the fiber/starch plastic had tensile strength i n the range of cellulose acetate, high-density polyethylene and polypropylene (Table IV). Elongation to break, which was low for the fiber/starch plastic, was comparable only to polystyrene. Tensile modulus was comparable for the fiber/starch plastic and cellulose acetate (Table TV). A s mentioned earlier, embrittlement of pregelatinized starch products could be due to moisture loss, structural relaxation, or crystallization (26). The X-ray diffraction pattern of newly formed starch plastics was similar to that of dried gels that had not been aged (compare Figures 4&7). However, the aged starch plastics


53

5

15

25

35

2 Thêta Figure S. X-Ray diffiactograms of a dried wheat starch gel and a new and aged (3 years) wheat starch foam. The curves were purposely offset to facilitate visual comparisons.


54

30

Ί

0

2

4

6

8

10

Strain (%) Figure 6. Typical tensile stress/strain curves for wheat starch plastics with and without softwood fiber reinforcement


55

5

15

25

35

2 Thêta Figure 7. X-Ray diflractograms of a dried wheat starch gel, a new wheat starch plastic and two aged (3 years) wheat starch plastics. The aged starch plastics gave two distinct X-ray patterns. The curves were purposely offset to facilitate visual comparisons.


56 had a diffraction pattern that deviated from the typical B-type pattern observed for the starch foams and aged, dried gel (Figure 7). The diffraction pattern for the aged starch plastic was variable. One pattern observed was similar to a B-type pattern except that the peak near 20.1 degrees was larger than the peak at 17.5 degrees. A second pattern was observed for the aged starch plastic that had several new peaks not apparent i n other starch diflractograms. The increase i n the ciystallinity of the starch plastics over time could partly account for the changes observed i n tensile properties. More research is needed to determine why two different diffraction patterns developed and whether the crystalline regions observed i n the starch plastics are unique.

Table Π Ι Tensile strength, modulus, elongation to break and toughness of plastics made of wheat starch or wheat starch plus 33% softwood fiber. The samples consisted of a newly made starch plastic, a starch plastic that had been aged for 3 years at room temperature and 50% relative humidity, and a newly made starch plastic containing fiber.

Sample

Tensile

Elongation

Tensile

Toughness

Strength (MPa)

To Break (%j

Modulus (MPa)

(MPa)

1.6a

900a

0.36a

a

Starch Plastic

12.4a

Starch Plastic

11.6a

0.92b

1230b

0.16b

27.5b

4.7b

1383c

2.3c

(Aged) Fiber/Starch Plastic

'Values within columns followed by a different letter are significantly different at the 95% confidence level.

The commercial potential of starch plastics is of wide interest. One application of particular interest is for microwavable food trays. There are concerns that plasticizers, unreacted monomers and other contaminants of plastics currently used for microwavable food trays can be absorbed by foods during baking creating a safety issue (33-35). The starch plastics were tested i n microwave cooking studies using low moisture, high moisture, and oily foods (23). The starch plastics had some properties such as high initial tensile strength, good appearance and desirable food safety qualities. However, the functional properties of the starch plastics were


57 inadequate for cooking applications. The starch plastic had a tendency to blister and deform during microwave baking. A starch copolymer with less hydrophilic properties would be better suited for this application.

Table IV. Typical values for tensile strength, modulus and elongation to break of fiber/starch plastics and various commercial plastics.

Tensile Strength

Elongation

Tensile

(MPa)

To Break

Modulus

(%)

(MPa)

27.5

4.7

1383

Cellulose Acetate

21

40

1300

Polyethylene - LD

8.3

550

180

Polyethylene -HD

29

60

830

Polystyrene

52

2.0

3450

Polypropylene

36

300

916

Sample

Fiber/Starch

The commercial potential of the starch plastics is largely determined by the cost of the final product. There may be niche applications where the starch plastics could function well. However, unless the product costs are comparable with the cost of commercial plastics, the starch plastics will not succeed in the marketplace. The raw material costs for making starch foams is relatively small but the processing costs are considerable. A more cost efficient way of making the starch foams is by making small beads as described earlier. The beads can be made using a continuous process and have a much shorter processing time than the slab gels. The beads could function as loose-fill insulation or be molded into foamed articles. One method used commercially for molding foamed beads into cups or bowls requires that the foam bead be expandable when it is heated. Polystyrene cups and bowls are made from pre-expanded polystyrene foam beads. The beads are pneumatically transported into a mold in which they are further expanded with heat The heat also makes the beads fuse together as they expand and fill the void space in the mold. The starch foam beads have an appearance very similar to that of expanded polystyrene beads. However, when starch foam beads are heated in a cup


58 molder they do not expand and fuse together as do the polystyrene foam beads. The reason for the failure of the starch beads to expand is readily apparent from S E M pictures of beads (Figure 2). The starch foams have an open cell structure with fibrous cell walls. In addition, the starch foam does not melt i n the temperature range of PS. Consequently, the starch foam beads do not expand or fuse when heated i n a cup molder. Further research into starch blends or starch copolymers could result i n the production of foam beads that can be processed similar to PS foam beads. The starch foam beads had other unique physical and mechanical properties that could be utilized for other applications. For instance, as mentioned earlier, the starch foams had a very fine microstructure. Scanning electron micrographs (Figure 2b) showed that the wheat starch foam had a high percentage of pores smaller than 1 μιη with some as large as 2-3 μιη. The lower range i n pore size was too small to determine from S E M micrographs and required porosimetiy analysis to determine the pore size range. The total pore volume of the foams was 3.23 ml/g. The median pore diameter was 0.616 μιη and the mean pore diameter was 0.065 μπι based on pore volume. Measurement of pore sizes i n the 5-14 Angstrom range revealed that the foams had a cumulative pore volume of 0.004 ml/g attributable to pores of 5 Angstroms i n diameter and a cumulative pore volume of 0.06 ml/g attributable to pores smaller 14 Angstroms or less i n diameter. The volume of extremely small pores i n the starch foams seems to confer chemical adsorption properties that could have commercial interest. The results indicated that the adsorption of non-polar compounds such as decane was low compared to charcoal and Tenax (Table V ) . The adsorption of 2-octanone, a polar compound, was higher for the starch foam beads than for charcoal. The starch foam beads were also very effective i n reducing the vapor pressure of alkylpyrazines such as 2-ethyl-3-methylpyrazine (Table V ) . There is also some evidence the foams may adsorb and help stabilize chemically unstable compounds. Table V . V a p o r pressure depression of three volatile chemicals (5 μΐ) sealed i n a flask with 0.5 g of wheat starch foam, charcoal or Tenax. Starch

Charcoal

Tenax

Decane

1.4

152

36

2-octanone

77

42

140

2-ethyl-3 -methylpyrazine

150

-

-

Compound

The small pore size of the foams appears to be a critical factor i n chemical adsorption since starch powder alone did not depress the vapor pressure. Popped


59 corn, a starch foam with large pore size, was also ineffective in lowering the vapor pressure. The volatile compounds were releasedfromthe starch foam beads when the structure was disrupted by the addition of water. The results indicate that the starch beads could have commercial value as a chemical absorbent. Applications in the food industry may include using the starch foam beads, which are food grade, as a carrier of flavor compounds in dry food products that are reconstituted in water.

Conclusion Starch can be processed into microcellular foams by air-drying starch gels that have been equilibrated in ethanol. The foams have low thermal conductivity, high compressive strength and have useful chemical absorbency properties. The foams can also be pressed into a plastic or afiber/starchplastic iffiberis incorporated in the formulation. Small spheres made in a continuous process may minimize the fabrication cost of the foams and be commercially viable.

References 1.

2.

3.

4.

5. 6. 7. 8.

9.

Whistler, R . L . In Starch Chemistry and Technology; Whistler, R . L . ; Bemiller, J. N . ; Paschall, E . F . , Eds.; Academic Press: New York, N Y . , 1984, pp 1-9. U.S. Congress, Office of Technology Assessment, Biopolymers: Making Materials Nature's Way, OTA-BP-E-102; U.S. Government Printing Office: Washington D C , 1993; pp19-50. Kirby, K . W . In Developments in Carbohydrate Chemistry; Alexander, R. J.; Zoebel, H . F . , Eds.; American Association of Cereal Chemists: St. Paul, M N , 1992, pp 371-386. Otey, F . H . ; Doane, W . M. In Starch Chemistry and Technology; Whistler, R . L . ; Bemiller, J. N . ; Paschall, E. F . , Eds.; Academic Press: New York, N Y . , 1984, pp 389-414. Koch, H . ; Roper, H . Starch 1988, 40, 121-131. Roper, H . ; Koch, H . Starch 1990, 42, 123-130. Bushuk, W. Cereal Foods World, 1986, 31, 218-226. Griffin, G . J. L. In Chemistry and Technology of Biodegradable Polymers, Griffin, G . J. L., E d . ; Blackie Academic & Professional: New York, N Y . , 1994, pp 135-150. Chinnaswamy, R. Hanna, M. A . Journal of Food Science 1988, 53(3), 834836, 840.

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Chinnaswamy, R. Hanna, M. A . Cereal Chemistry 1988, 65(2), 138-143.

11. 12.

Tiefenbacher, K . F. J.M.S.-Pure Appl. Chem. 1993, A30(9 & 10), 727-731. Hilyard, N . C.; Young, J. In Mechanics of Cellular Plastics, Hilyard, N . C . , Ed.; Macmillan Publishing Co.: New York, N Y . , 1982, 1-26.


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